The present invention relates to sports shoes, and in particular, to a running shoe constructed to minimize impact shock and to maximize lateral stability.
Extensive clinical evaluation of foot and knee injuries sustained by runners and joggers suggests that the most important factors associated with such injuries are shock absorption on impact and lateral foot stability. Based on injury data, these two factors appear to be of about equal importance. Therefore, both factors should be considered in proposing improvements in sports shoes.
For most runners, initial foot impact occurs in the heel region. Heel cushioning material, which is contained principally in the shoe's midsole of a running shoe has a firmness which provides proper impact cushioning for a person of about average weight. Where the runner is quite heavy, the heel cushioning material may "bottom out" before heel impact is completely absorbed, and shock-related injuries can result. On the other hand, poor lateral foot stability may result in conventionally constructed running shoes if the cushioning material is too soft.
Lateral foot stability refers to a shoe's ability to control the normal tendency of a foot to roll toward its inside on impact. Ideally this inward rolling of the foot, which is known as pronation, is arrested about when the knee is maximally flexed. Where the foot continues pronation after the knee reaches its maximum flexion and has begun to straighten, cumulative knee strain leading to knee injury may occur. As a general rule, prior art running shoes having a relatively firm midsole, particular in the heel region, provide the best lateral stability.
One general object of the present invention is to provide a running shoe having both good shock absorption and lateral stability characteristics.
A more specific object is to provide such a shoe in which the runner's weight, during initial impact, is transferred from softer to firmer cushioning material, to provide effective shock absorption in both light and heavy runners.
Still another object is to provide such a shoe which is constructed to increase lateral foot stability without sacrifice in shock absorption characteristics.
The sports shoe of the present invention includes a sole layer which is formed of an inner side layer portion having one overall firmness and an outer side layer portion having a lesser overall firmness. In one embodiment of the invention, the layer portions are located substantially in the heel region of the layer. In another embodiment, such portions extend substantially throughout the length of the layer.